1. Field of the Invention
The present invention relates to a hybrid disc of a dye membrane recording type having a read-only area and a recordable area.
2. Description of the Related Art
A read-only optical disc, such as a CD (Compact Disc) of a read only type has become widespread. Such a read-only optical disc has phase pits to record information. Each phase pit is formed such that its depth is equal to one-four wavelength .lambda. of a light beam used for reproduction, i.e., .lambda./4.
On the other hand, a recordable optical disc is developed. Especially, as one type of the recordable optical disc, a write once type optical disc, such as a CD-R (CD-Recordable) and the like is frequently developed. Such a write once type optical disc has a recording layer composed of a dye membrane. When information is recorded on the write once type optical disc, a light beam modulated such that its intensity is changed depending on the information is emitted onto the recording layer. Thereby, reflectance of the dye membrane at the irradiated part is changed. Thus, the information is recorded on the disc. When reproducing, the recorded information is reproduced on the basis of a change of reflectance.
Here, in the above mentioned write once type optical disc, the intensity of the light beam used for recording is alternatively changed between a first intensity and a second intensity. Namely, when the information to be recorded onto the disc is a high level, the intensity of the light beam is the first intensity. When the information is a low level, the intensity of the light beam is the second intensity. The second intensity is weaker than the first intensity, and the second intensity is roughly or accurately equal to the intensity of a light beam used for reproduction. When the information is recorded on the disc, reflectance of the dye membrane is changed by switching over the intensity of the light beam between the first intensity and the second intensity. In order to increase the intensity of the light beam, it is preferable that the light beam is efficiently used.
Furthermore, when the information is recorded on the write one type optical disc, it is necessary that the light beam is accurately emitted on the target track in order to record the information on it accurately. Therefore, a tracking control is performed. Various kinds of methods to perform the tracking control have been developed. In view of the efficiency of the light beam, a push-pull method is preferably used as the tracking control for recording information on the write once type optical disc. In the push-pull method, a single light beam is used for both recording and the tracking control. In case that the tracking control is performed by using the push-pull method, in order to increase a level of a tracking error signal to the maximum, it is necessary that height (depth) of the information track of the write once type optical disc is one-eight waveform .lambda. of the light beam, i.e., .lambda./8.
Therefore, in the write once type optical disc, in view of both recording and reproduction, a predetermined value between .lambda./8 to .lambda./4 is set as the depth of the information track.
Here, in view of the recent development of multimedia, an optical disc having the function of the read-only optical disc and the function of the write once type optical disc is desired.
However, it is difficult to realize such an optical disc (Hereinafter, it is referred to as a "hybrid disc".). The reason is that the depth of the phase pit of the read-only optical disc is different from that of the information track of the write once type optical disc. Namely, the depth of the phase pit of the read-only optical disc is .lambda./4, but the depth of the information track of the write once type optical disc is the predetermined value between .lambda./8 to .lambda./4 (not .lambda./4). It is very difficult in view of a yield to form the phase pit and the information track each having different depth on a single disc.